A static class AB amplifier is widely used in filter design because of its good efficiency to drive load. However, the static class AB amplifier generally has a stage positioned before an output stage to set appropriate DC levels to the output stage, and because a transconductance and an output resistance of this stage vary with a signal swing, the variations of the transconductance and the output resistance will generate distortion. The distortion cannot be suppressed by loop gain at high frequency, meaning that a linearity of the class AB amplifier is dominated by the distortion/nonlinearity of this stage. In addition, because a third-order intermodulation distortion (IM3) has a strict requirement in Federal Communications Commission (FCC) specification, improving the distortion/nonlinearity of the class AB amplifier to meet the FCC requirement is important.